The invention relates to a method for detecting a failure in a three-phase alternator, and an associated failure detection device.
In particular, this detection method is adapted to detect an open circuit in a phase of the alternator and/or a short-circuit of a phase to ground.
This method may, for example, be used to detect failure of an alternator supplying a control unit of an engine, for example an engine of an aircraft. Such a control unit controls the operation of the engine according to the altitude, external pressure, external temperature, flow rate of the injected fuel, etc. In such an application, for passenger safety it is important to detect any failure of the alternator in order to quickly establish a backup power mode. The method and device for detecting failures may, however, be used in any other application where a three-phase alternator is used to supply power.
The failure detection is usually performed by measuring the phase shift between the three voltages delivered by the alternator. In the absence of failure, there is a 120° phase shift between each of these voltages. In the event of an open circuit or a short-circuit to ground, these phase relationships change. The phase shift is measured using voltages sampled at the diodes of the three-phase rectifier bridge, reshaped, and compared to reference levels. Logic signals representative of each phase are then analyzed by counting, directly by a programmable logic circuit or by an internal counter of the microprocessor.
However, the method of detecting a failure by measuring the phase shift does not detect all types of short-circuit to ground and all types of open circuit. Indeed, to protect electronic circuits from electromagnetic exposure (compliance with electromagnetic compatibility (EMC) standards—lightning), capacitors are mounted between each phase and the ground. If one phase is cut off, the corresponding input from the rectifier bridge still remains connected to the rest of the circuit via the resulting serial-parallel combinations of these capacitors. A residual voltage therefore appears at the corresponding input to the phase-shift measurement device. Under a high alternator load, this residual voltage may be higher than the normal voltage generated by the alternator when under a low load, and may be sufficient to activate the phase measurement circuit. Accordingly, this method of detecting a failure by measuring the phase shift does not allow differentiating a case of an open circuit under high load from a case with no failure when under a low load.
In addition, in case of a short-circuit to ground of one of the phases, common mode current flows in the current return wires and in the remaining phases, and causes a voltage drop proportional to the wire length. If the alternator and battery used in the alternator start-up stage are more than a few meters away from the load (for example, the engine control unit), the resulting voltage drop will disrupt the phase shift measurement.
Finally, the phase shift measurements are measurements made over time using a microprocessor or a programmable logic circuit. Such components are costly. In addition, the use of these components requires certifications which are also costly.
The object of the present invention is to provide a detection method and device for detecting an open circuit in one of the phases regardless of the presence of protective components which enclose the circuit and regardless of the operating conditions of the alternator.
Advantageously, the failure detection method and device of the invention are capable of detecting a short-circuit to ground, regardless of the length of the wiring.
Advantageously, the detection device can be implemented both with analog components or software components.
To this end, the invention relates to a method for detecting a failure of a three-phase alternator connected to a three-phase rectifier bridge; said method comprising the steps of:                rectification of the voltages generated by the three-phase alternator in a first phase, a second phase, and a third phase,        addition of the rectified voltage sampled from the first phase, to the rectified voltage sampled from the second phase, in order to obtain a voltage sum;        subtraction, from said voltage sum, of twice the rectified voltage sampled from the third phase, in order to obtain a voltage difference;        comparison of the voltage difference to a high threshold and to a low threshold; and        detection of a failure when the voltage difference is less than said low threshold or when the voltage difference is greater than said high threshold.        
According to some particular embodiments, the detection method comprises one or more of the following features:                it further comprises a step of filtering the rectified voltages sampled from the first, second, and third phases, said filtering step occurring prior to said addition and subtraction steps.        it further comprises a step of attenuating the rectified voltages sampled from the first, second, and third phases, said attenuation step occurring prior to said addition and subtraction steps.        
The invention relates to a device for detecting a failure of a three-phase alternator, said detection device comprising:                a three-phase rectifier bridge connected to a first phase, second phase, and third phase of said three-phase alternator;        a summation and subtraction unit adapted for adding a rectified voltage sampled from the first phase to a voltage sampled from the second phase in order to obtain a voltage sum; said summation and subtraction unit being adapted for subtracting, from said voltage sum, twice a voltage sampled from said third phase, in order to obtain a voltage difference;        a window comparator adapted for comparing said voltage difference to a high threshold and to a low threshold;        a monitoring unit adapted for transmitting a failure signal when said voltage difference is less than the low threshold or when said voltage difference is greater than the high threshold.        
According to some particular embodiments, the detection device comprises one or more of the following features:                said summation and subtraction unit comprises an amplifier having an inverting input and a non-inverting input, the inverting input being connected to the first phase via a first resistor and to the second phase via a second resistor, the non-inverting input being connected to the third phase via a third resistor; and the value of the first resistor is equal to the value of the second resistor, and the value of the third resistor is equal to half the value of the first resistor.        said window comparator is a window comparator with hysteresis.        said window comparator with hysteresis comprises a first comparator and a second comparator each having an output; a first isolating diode being connected to the output of the first comparator and a second isolating diode being connected to the output of the second comparator.        it comprises an earth ground, a first, a second, and a third filter circuits adapted for filtering the voltages generated by the three-phase alternator; the first, second, and third filter circuits being connected between said earth ground and the first, second, and third phases respectively.        it comprises an earth ground, and first, second, and third attenuation circuits adapted for attenuating the voltages generated by the three-phase alternator; the first, second, and third attenuation circuits being connected between said earth ground and the first, second, and third phases, respectively.        
Finally, the invention relates to a computer program comprising instructions for implementing the method mentioned above, when executed by a processor.